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A247395 The smallest numbers of every class in a classification of positive numbers (see comment). 6
1, 2, 4, 10, 26, 50, 122, 170, 290, 362, 530, 842, 962, 1370, 1682, 1850, 2210, 2810, 3482, 3722, 4490, 5042, 5330, 6242, 6890, 7922, 9410, 10202, 10610, 11450, 11882, 12770, 16130, 17162, 18770, 19322, 22202, 22802, 24650, 26570, 27890, 29930, 32042, 32762 (list; graph; refs; listen; history; text; internal format)
OFFSET

0,2

COMMENTS

Consider a classification of the positive numbers with classes {1}, A000040 (primes), A156759 (n>=2) (preprimes, or preprimes of the first kind), A247393 (preprimes of the second kind), A247394 (preprimes of the third kind), etc.

Then a(0)=1, a(1)=2; for n>=3, a(n) is the smallest number which is a preprime of the (n-1)st kind.

LINKS

Indranil Ghosh, Table of n, a(n) for n = 0..1000

Vladimir Shevelev, A classification of the positive integers over primes

FORMULA

For n>=3, a(n) = (prime(n-1))^2 + 1.

MATHEMATICA

Table[If[n>2, Prime[n - 1]^2 + 1, 2^n], {n, 0, 43}] (* Indranil Ghosh, Mar 08 2017 *)

PROG

(PARI) a(n)=if(n>2, (prime(n-1))^2 + 1, 2^n) \\ Charles R Greathouse IV, Sep 17 2014

CROSSREFS

Cf. A000040, A156759, A247393, A247394.

Sequence in context: A111564 A007021 A100605 * A183947 A154322 A090031

Adjacent sequences:  A247392 A247393 A247394 * A247396 A247397 A247398

KEYWORD

nonn,easy

AUTHOR

Vladimir Shevelev, Sep 16 2014

EXTENSIONS

More terms from Peter J. C. Moses, Sep 16 2014

STATUS

approved

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Last modified July 4 14:51 EDT 2020. Contains 335448 sequences. (Running on oeis4.)